The horn shark, Heterodontus francisci, occurs in the eastern Pacific Ocean from central California to the Gulf of California, and probably off Ecuador and Peru. It is a peculiar looking shark, with brow ridges, fin spines, and a dense coat of smooth placoid scales. This fish lays large spiral-shaped egg capsules, each containing a single embryo.

Its diet is odd as well: horn sharks specialize in eating hard prey items like sea urchins (see Strongylocentrotus purpuratus). The scientific name Heterodontus means different teeth. This name is appropriate because, unlike most sharks which have only pointy cutting teeth, horn sharks have pointy teeth at the front of their jaws and rounded molar-like teeth at the back. The pointy teeth are used for grasping prey, and the molar-like teeth are used for crushing hard prey.

The skeleton of cartilaginous fishes like Heterodontus francisci is composed of cartilage. The surface of many of the skeletal elements is calcified into tiny mineralized blocks called tesserae; if not for this calcification, we could not generate a clear CT image of a cartilaginous fish skeleton. Crushing hard prey with a cartilaginous skeleton is no easy trick, and the stingrays that specialize on clams and mussels (e.g., Aetobatus narinari have developed calcified struts that run right through the jaws to support the crushing surfaces. The scan of the adult horn shark was conducted to determine whether they have this strut-reinforced cartilage, or if they had found some other solution to eating hard prey. As you can see from the movies above, horn sharks have no calcified struts running through their jaws. The CT scan allowed us to determine that it is the shape of the jaws, not extra mineralization, that allows them to crunch and munch.

This specimen of Heterodontus francisci (AL21), an adult female (58.5 cm long), was made available to The University of Texas High-Resolution X-ray CT Facility for scanning courtesy of Dr. Adam Summers of the Department of Ecology and Evolutionary Biology, University of California, Irvine. Funding for scanning was provided by support from the McDowell Foundation to Dr. Summers and by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin.

About this Specimen

The specimen was scanned by Matthew Colbert and Richard Ketcham on 19 May 2000 along the coronal axis for a total of 364 512x512 pixel slices. Each slice is 0.25 mm thick, with an interslice spacing of 0.25 mm and a field of reconstruction of 105 mm. The dataset displayed was reduced for optimal Web delivery from the original, much higher resolution CT data.